U.S. Pat. No. 3,761,488 describes and claims a class of isothiazolone compounds, which comprises both components of MIT.
U.S. Pat. No. 3,849,430 describes and claims a process for preparing the said compounds by halogenation, accompanied by cyclization, of disulfideamides or mercaptoamides.
U.S. Pat. No. 4,105,431 discloses and claims the same compounds as compositions for inhibiting the growth of bacteria, fungi or algae, and a method for inhibiting said growth by the use of said compounds. EP 95907 claims a biocidal composition containing 3-isothiazolones, which is characterized in that the impurity content in the composition of nitrosamines plus nitrosamine precursors is less than 15 ppm of the 3-isothiazolones.
EP 363011 claims microbiocidal compositions of MIT in which the percentage of 5Cl-MIT is from 1.2 to 25.4%, the remaining part of the composition being 5H-MIT. Increased biological activity over that of each material separately is stated to be possessed by the compositions, which are also said to produce less skin-sensitization than compositions containing higher amounts of 5Cl-MIT. The compositions are prepared by semi-batch reaction between N-methyl-3-mercaptopropionamide (MMPA) and Cl.sub.2 in monochlorobenzene, the molar ratio of Cl.sub.2 to MMPA being 2.
EP 375265 claims a process for the preparation of crystalline isothiazolone monohydrochloride salts by chlorination of disulfidediamides or mercaptoamides, to give a crude product which is purified by crystallizing from chlorobenzene via reduction of the HCl content.
All the processes described in the literature, and in particular in the aforementioned patents, are batch or semi-batch. One of the advantages of MIT as an industrial biocide is that it rapidly decomposes in the environment and therefore does not constitute an ecological hazard. On the other hand, its shelf-life and that of its formulations can be seriously shortened by even traces of destabilizing impurities. Furthermore, the chemistry involved in MIT manufacture is multi-step and very complex. The composition of the product, the level of impurities and, as a result, the stability of formulations containing MIT, can be radically affected by even minor process fluctuations. For this reason, batch or semi-batch operations have been found to produce products of variable and inferior quality.
In addition, and in spite of the advantages claimed for MIT compositions in which the 5Cl component is in the minority, as in EP 363011, the most useful compositions are those in which the 5Cl/5H ratio is 2.5 or higher. Batch and semi-batch operations directed to produce mixtures of the said high 5Cl/5H ratios have the disadvantage of causing a large loss of yield because of the formation of large amounts of overchlorinated materials (mainly 4,5-dichloro-MIT).
It is a purpose of this invention to provide a process for the manufacture of MIT mixtures that will be free of the drawbacks of known processes, and will afford high yields.
It is another purpose of the invention to provide such a process which will permit to manufacture MIT mixtures in which the chlorinated and unchlorinated component are present in any desired molar ratio, without loss of yield to undesired by-products.
It is a further purpose of the invention to provide such a process which will permit preparation of MIT mixtures in which the 5Cl-5H ratio is greater than 2.5, and which do not cause large losses of yield due to the production of overchlorinated materials.
It is a still further purpose of the invention to provide such a process which will yield solid MIT product as its white HCl-salt (MIT.HCl) which is adequately stable at temperatures convenient for shipping in such a form.
It is a still further purpose of the invention to provide such a process that can be carried out efficiently and with high yields by means of the same apparatus, no matter what the desired ratio of the components of the MIT mixture may be.
Other purposes and advantages of the invention will appear as the description proceeds.